This invention relates to phase shifting and more particularly to a precision phase shifter having improved accuracy and reduction of complexity.
Note: Applies to fiber optic networks or any broadband, high data rate communication. In fiber optic cable networks, or indeed in any broadband high data rate communications system, there is a requirement to regenerate or amplify the signals along a cable path so as to provide robust communication from a head-end system to a destination or receiver. With transmission rates presently exceeding 10 gigabits/sec., loss of timing results in lost data or degration of data integrity when the pulse trains are transmitted over long distances. In such a high data rate system the behavior of electric circuits is quite similar to microwave circuits operating in the 5 GHz range. Thus, part of the loss of data can be viewed as the result of a phase shift between a data pulse train and a clock pulse train due to the dispersion which occurs in fiber optic cables over long distances. Loss of data can also be due to loss of signal amplitude over the distance and degradation of the signal shape such as broadening of the signal pulse due to dispersion inside the optical cables.
The problem involved in the transmission of data streams over long distances is thus the fact that the data streams themselves degrade such that not only is the amplitude of the signals reduced with distance, the waveforms which are originally squared up become degraded. As a result, in some instances the degraded pulse trains cannot be appropriately sampled, retrieved and recovered due to a phase offset in the timing between timing pulses sent at the same time as the data stream.
In the past, the degradation due to dispersion and other artifacts was such that regeneration units needed to be dispersed along the transmission line every 70 to 100 miles. The reason that the regeneration units were necessary was to shape up the pulses that had degraded and to realign them with respect to a timing reference. It will however be appreciated that regeneration units are expensive and in general require an optical filter, a photo detector and an amplifier along with a data-recovery system which after the pulses are squared up are adjusted in time to realign them with a recovered clock signal. Regeneration usually requires converting the optical signals back to electronic, then amplifying and re-shaping, and then converting them into optical signals again. While some regeneration uses all-optics amplifiers without converting into electronic signals and back, there is still the need for electric power to operate the regeneration systems.
In the past, one method of recovering the data involved shifting the entire data stream either forward or backward so that the phase delay between the clock stream and the data stream could be adjusted by the operator for the entire system or in some cases reduced to zero. Note that the amount of shift can be under operator control to achieve desired alignment between data and clock pulse. This was accomplished by dialing in a particular phase shift to shift the data pulse train such that the leading or trailing edges of the clock and data pulses would be aligned.
Phase shift, as used herein, refers to the offset between either leading or trailing edges of a clock pulse and those of the data pulse train.
In order to shift the pulse train by for instance a full 360xc2x0, it was a practice to serially connect or cascade a number of phase shifting modules, that incrementally phase shifted the input signal as it passed through the various phase shifting modules. The desideratum was to be able to exercise phase shift control to one degree of resolution over the entire 360xc2x0.
This required a large number of phase shifting units of different phase shift angles whose combinations could give any angle between zero and 360xc2x0. However, when phase shift modules were serially connected together in a reasonable package, cross talk between the phase-shifting modules, particularly among the control wires to the modules resulted in a practical limit of phase shift resolution to about 12xc2x0+/xe2x88x923xc2x0 for a bit rate of nominally 10 gigabit/sec, or frequency around 5 GHz.
It will be appreciated that a 12xc2x0 phase shift is unacceptable in terms of the precision by which the pulse train needed to be shifted with respect to the clock pulse train, and as such, resulted in the inability to eliminate the costly regeneration units.
As will be seen, key to the subject invention is the recognition that the cause of the phase shift errors is the many pairs of control wires to the phase shifting modules inside a compact container, which cause electromagnetic interference with one another in an unpredictable manner. Note that the control wires carry electric voltages that either activate or de-activate a particular phase shifting module. The use of the multiple pains of control was led to the realization that with the presently available phase shifting modules, it would be impossible to reliably control phase shifting to the accuracy required for eliminating the regeneration units in for instance a fiber optic cable system stretching for 2500 miles.
Secondly, in the prior art, two voltages on 2 pairs of control wires were utilized for the phase shifting modules to switch in and out two different electrical paths of differing length. In these phase shifters it was found that the two voltages must switch in perfect sync in order for the phase shifts to be reliably produced.
In point of fact, however, the two control voltages have different voltage rise and fall times which are the result of the internal capacitance of the circuit, such that by utilizing the two voltages and having any mismatch in the rise and fall times, phase shift errors occurred due to mismatch in the transitions in the two voltages.
While it will be noted that dimensional changes of connectors and circuit elements inside the entire phase shifter unit due to temperature changes result in systematic errors which can be characterized by testing and therefore can be compensated, cross talk between the many pairs of control wires results in random errors that cannot be systematically compensated, thus, still presenting a problem.
The result of the inability to control the phase shift resolution to only 12xc2x0 at 5 GHz prevented long haul fiber optic cable plants from eliminating the number of regenerators required. The inability to shift the received data pulse trains to the required accuracy thus severely limited the ability to provide a cost-effective high-speed fiber optic network.
It has now been found that in a fiber optic transmission system having a 10-gigabit/sec bit rate a 3xc2x0+/xe2x88x921xc2x0 resolution is achievable using binary combinations of control voltages in which only one voltage and only one pair of control wires is utilized for the individual phase shifting modules of the phase shifter over the entire range of 360xc2x0. It has been found with the subject techniques that each of the phase shifting modules of seven different angles, namely 180xc2x0, 90xc2x0, 45xc2x0, 22.5xc2x0, 11.25xc2x0, 6xc2x0 and 3xc2x0, can achieve a phase shift of any multiple of 3xc2x0 with an accuracy of +/xe2x88x921xc2x0. What this accuracy means is that a number of regeneration units can be eliminated. In one embodiment, pulse trains were recovered when transmitted over 2500 miles without regeneration along the way. Note that in one embodiment a 180xc2x0 phase shift is achieved by cascading two 90xc2x0 phase shift modules.
As mentioned above, this accuracy is achieved through the utilization of only one control voltage, which requires only one pair of wires for each module. The result is that the subject systems succeed in reducing the number of control wires by 50%, and consequently the amount of interference among the seven phase shifting modules inside the phase shifter. Additionally, there is no unwanted error due to the transitioning of the control voltage because there is only one control voltage utilized, versus two in the traditional design.
In one embodiment the phase shifting modules utilize PIN diodes as switching devices to reroute the microwave clock and data streams, which fall under microwave frequency regime, to change its propagation length and therefore its propagation delay and phase. In a preferred embodiment, a single control line is utilized to carry a bipolar control voltage to switch the signal state of the phase shifting module. For instance, a positive control voltage for one single phase state, and a negative signal control voltage for the other signal phase state causes the signals to go through one of two paths through the phase shifting module. For each signal path in the phase shifting element, there are two PIN diodes, a resistor and a capacitor, in its simplest form.
Thus, each phase shifting module comprises two signal paths of different length to provide a predetermined value or difference. The microwave input signal is switched between the two paths to obtain the desired phase shift.
In one embodiment for each phase shifting module there is one DC blocking capacitor for the input and one for the output, with the control voltage introduced through an RF choke in one end and returned to ground through another RF choke in the other end.
It will be appreciated that the PIN diodes can be completely turned on by forward-biasing with a forward current and turned off by reverse-biasing it.
The result is that a simplified physical arrangement is achieved for the control lines to an individual phase shifting module, which minimizes the interference of the DC control lines with respect to the phase shifting performance. Moreover, since the control voltage is switched from positive to negative in a single continuous sweep without complicated crossover control circuits, smooth phase shifting can be achieved. This is because there is only one rise and fall time for the control voltage. Since there is no interplay between the rise and fall times of two separate control voltages as was the case in prior art systems, timing and phase errors associated with the non-synchronized voltages are eliminated.
The result of cascading the phase shifting modules is that for an equivalent to a 5 GHz microwave system, a more precise ultimate phase shifting result is achieved. Being able to phase-adjust the data pulse train vis-xc3xa1-vis the clock pulses in such a precise manner permits the removal of regeneration units normally thought to be required in long-haul fiber optic cable systems. Being able to adjust data pulse train vis-xc3xa1-vis the clock pulses in such a precise manner in a 10 gigabit/sec system provides a key functional capability to xe2x80x9clong haulxe2x80x9d optical systems such that the distance between regeneration is now substantially increased. For instance, in a 2500 mile length of cable, it has been found that one needs only a preamplifier and squaring circuit at the receive end, followed by the ability to phase shift the recovered pulse train to a 3xc2x0 accuracy which maintains acceptably low error rates in the data transmission.
In summary, an improved accuracy phase shifter having cascaded phase shifting modules is provided for phase shifting a pulse train which rather than utilizing two pairs of control voltage leads for each phase shifting module, utilizes only one pair to significantly reduce the interference between the cascaded phase shifting modules. The result is that the phase of an incoming pulse train can be shifted from 0 to 360xc2x0 in 3xc2x0 increments with +/xe2x88x921xc2x0 accuracy. When the subject phase shifter is utilized in fiber-optic cable networks operating at 10 gigabits/sec. for correcting phase offsets of data streams transmitted over ultra long distances, the utilization of the subject phase shifter plays a part in obviating the necessity of having regeneration units every 70 to 100 miles, thus permitting error-free transmission over as many as 2500 miles without the requirement for regeneration.